I don't think there's any canon standard for "bastard's names". Here are a few things you might consider:
- Mother's name - She's a bastard, not an orphan, so why not give her the same last name as her mom?
- Father's name - why not? If C'kayah had an illegitimate child, there's no reason she couldn't be C'saber Kayah...
- Profession - Say Saber becomes a blacksmith. She could be Saber Smith.
- Self-created - Saber Starlörd sounds pretty badass
- Pseudonym - Blade Cutts - you see what I did there?
- Adoptive name - Saber is adopted by C'kayah, so she becomes Saber Kayah.
- Orphanage name - Saber is thrown out in the streets and taken in by the Lulubelle orphanage, becoming Saber Lulubelle.
- Title - Maybe Saber just goes as Saber, and you repurpose the last name to be a title she gives herself: Saber Boldaxe.
